1. Field
The invention is in the field of rubber-tired wheels for and in combination with industrial vehicles known as "roof bolters" as used in underground mines to drill holes and secure anchoring bolts in the roofs of mining and haulage drifts and tunnels to prevent accidental detachment and falling of overhanging slabs of the natural roof material.
2. State of the Art
Roof bolters are heavy, cumbersome vehicles having elongate chassis frames. The frame of such a vehicle is supported on four wheels mounted amidships and at opposite sides of the vehicle. A boom and cradle assembly is cantilevered forwardly of the front wheels, and a cable reel and drive mechanism rearwardly of the rear wheels. This results in an elongated vehicle with a short wheel base and extensive projections cantilevered at the front and rear ends. The wheels are individually secured to respective stub axles, which are journaled in the frame, and have respective sprocket and sprocket chain drives. The front wheels are fixed against being turned from side-to-side and are separately driven for steering purposes.
A typical roof bolter of this type is normally provided with 7.50.times.15 tires on fifteen inch diameter wheels having overall rim width of substantially eight and three-eighths inches, as original equipment. The full weight of the vehicle is thus borne by the seven and one-half inch width of the tires, so that the load per square inch of tread is high. Since the environment in which the wheels operate is very demanding, i.e. uneven, rough ground surface frequently strewn with lumps of ore or coal, depending upon the type of mine, careless handling, sudden starts and stops, jagged tunnel walls which tear the side walls of the tires, and twisting of the tires on the wheels as the vehicle is turned, make the effective life of these tires extremely short, e.g. on the order of one month or six weeks.
Efforts have been made to extend the lives of the tires and to minimize downtime by inflating them with a foamed plastic instead of air. However, due to the stress of the operating environment, the tires and the foam filler tend to slip and rotate on the wheels unles the foam is blown into the tire to a pressure of approximately 130 p.s.i. This over-inflation of the tires avoids relative slippage between the tires and wheels, but does not significantly extend the effective lives of the tires. In addition, at a pressure of 130 p.s.i., the foam-filled tires are like solid rubber tires, with litte or no give or compressibility. The result is that the tires do not effectively cushion the shocks from the rough, uneven, ground surface, but transmit them directly to the axles and thence to the vehicle frame, with consequent damage to the vehicle and the equipment mounted thereon. The amount of time during which the vehicle is out of serive due to tire damage or damage to the sprocket drives, axles, and other components has thus been inordinately large, and the expense of tire replacement, vehicle repair, and down time of the vehicle has been exhorbitant.